1. Field of the Invention
The present invention relates to devices, such as image forming devices, that include a fixing device that affixes toner, or another image forming substance, to a sheet so as to make a toner image on the sheet.
2. Discussion of the Background
Image forming apparatuses, such as electrostatic copying machines, printers and facsimiles that employ an electrophotography process also include a fixing apparatus that fixes a toner image on a transfer paper. A conventional fixing apparatus includes a heating roller having a heating element therein and a press roller that contacts the heating roller. The conventional fixing apparatus is adapted to pass the transfer paper between the heating roller and press roller such that a toner image disposed on the transfer paper becomes fixed to the transfer paper as a result of heat imparted to the toner by the heating roller and pressure applied to the toner and transfer paper by the press roller and heating roller.
A quality of the bond between the toner and transfer paper depends on heat conditions of the fixing apparatus. For example, as the toner is heated beyond a predetermined melting temperature, the quality of the fixing process improves because the toner melts well. However, if the toner is not heated above the predetermined temperature, the quality of the fixing process is sub-optimal because the toner only partially melts.
Japanese Laid-Open Pat. Application No. 53-50844 discloses an induction heating element in the form of a heating roller. As shown in FIG. 12, this heating roller includes a core 2 made of a magnetic material fixed to a shaft 1, a coil of wire 3 wound around the core, a roller member 5 which is an induction heating member rotatably supported by the shaft 1, and a heat-resistant and heat-insulating layer 4 arranged on an inner circumferential surface of the roller member. In the heating roller, a current (generally, 5 to 15 A) from a commercial power supply 8 is supplied to the coil via leads 6 and 7 to generate an induced current in the roller member 5. This induced current flows in the presence of an internal resistance in the roller member 5, which, according to the Joule effect, produces thermal energy, and thus heat, as a result of the induced current flow in the roller member 5.
In the induction heating system of FIG. 12, the coil 3 is arranged inside the roller member 5 and a high voltage is applied to the coil so as to supply a high current during a fixing operation in an attempt to heat the toner to a sufficient temperature. In addition, the roller member 5 covering the coil is made of a wire made of a conductive material having an internal resistance such that, when subjected to a high current, the wire itself produces heat, albeit a small amount. So respective windings in the coil 3 do not short-out to adjacent windings or to other conductive bodies, the wire is coated with an insulating layer. However, if a portion between the coil 3 is subjected to too much heat, there is a risk that a part of the insulating layer will deteriorate, thereby causing adjacent windings to short-out.
Generally, available insulating materials that are suitable for coating the wire are expensive, and the present inventors have identified that avoiding this expense by employing a structural alternative would be desirable, if possible. Furthermore, avoiding special steps for coating the wire with the special insulating materials would also be desirable.
FIG. 13 shows another conventional induction heating roller as disclosed in Japanese Laid-Open Patent Application No. 58-209887. This induction heating roller includes a hollow roller 231 and a supporting member 232 which supports the hollow roller 231. A solid core portion 234 is included and an induction coil 233 is mounted on an outer periphery of the solid core portion 234. A supporting shaft 236 which protrudes from each side of the core portion 234 rotatably supports a hollow shaft portion 238 of the hollow roller 231 via a bearing 237. Further, on the supporting shaft 236, there is provided a lead wire 239, one end of which is connected to the induction coil 233. The lead wire 239 is led out of the supporting shaft 236 to connect to a power supply (not shown). In addition, a jacket 241 is put on the supporting member 232 and a cylindrical thermal insulating material 242 is concentrically wound around the induction coil 233.
In this induction heating roller, a refrigerant is circulated through the jacket 241, as shown, to cool the supporting member 232, thereby preventing the induction coil 233 from receiving conduction heat from the hollow roller 231. In addition, the thermal insulating material 242 is used to intercept radiation heat and convection heat generated by the hollow roller 231, thereby preventing the induction coil 233 from being exposed to the heat. Thus, the induction heating roller of FIG. 13 addresses the concern of overheating the induction coil 233 by a combining an active cooling mechanism with sufficient thermal insulating material.
As recognized by the present inventors, the conventional heating roller of FIG. 13 is an expensive approach for solving the problem because this structure is complex in that (1) the thermal insulating material is wound around the induction coil, (2) the jacket is put on the supporting member, (3) the thermal insulating material is used and (4) the refrigerant is used. Another limitation with the device of FIG. 13, is that a copy operation start up period is relatively long because the refrigerant initially absorbs much of the thermal energy.
Another induction heating roller is disclosed in Japanese Granted Utility Model Application No. 57-52874, in which a supporting member is configured to support a hollow roller having an iron core therethrough. An induction coil is mounted about an outer periphery of the iron core, and the iron core supports the hollow roller via a bearing. At a precise location about the outer surface of the iron core, an electrical insulating spacer is provided for preventing a short-circuit to occur between the coil and the iron core. Other electrical insulating spacers, of a different type, are inserted about the core and between respective windings of the wire so as to prevent the windings from short-circuiting.
As recognized by the present inventors, a limitation with this conventional heating roller is that the process for forming the spacers on the heat roller is complex and thus expensive. Furthermore, this type of roller cannot be manufactured as quickly as other heating rollers, which is a significant manufacturing liability.
In the above-mentioned heating roller, the iron core is made of a magnetic material, although alternatively a bobbin made of a heat-resistant material may be used instead of this iron core. When the bobbin is used, and when the heating roller becomes hot, there is a risk that the bobbin shape will become deformed, perhaps in an eccentric shape. As a result of the deformation, a problem occurs in that the coil wound around the bobbin comes into contact with the roller member, thus resulting in the creation of an electrical leakage current or the like. The deformation problem becomes particularly pronounced when the induction heating roller is 15-50 millimeters in diameter and 1-2 millimeters in thickness and used in an image forming apparatus because it is difficult to maintain adequate gap-control between the hollow roller and the coil.
Furthermore, as the heating roller becomes bent as a result of pressure being applied thereto from the press roller, a problem occurs that, especially when the heating roller rotates at high velocity, the heating roller comes into contact with the coil, thereby resulting in electrical leakage.